Many telephone companies are beginning to use automated systems for achieving monitoring, testing, and management objectives for their fiber optic networks. In such automated systems, optical time domain reflectometers (OTDR) are employed for testing optical fibers for attenuation, discontinuities, and faults, which affect the quality of optical signals transmitted through the fibers. In testing an optical fiber with an OTDR, a fiber under test is connected to the OTDR and a laser is periodically energized to launch light pulses into the fiber under test. During the interval between the pulses, backscattered light from the fiber produced by the Rayleigh effect and reflected light produced by discrete reflection sites (e.g., splices) are directed to a photosensitive detector, such as an avalanche photodiode or the like. The detector converts the backscattered light signal into an electrical signal, which is amplified, sampled, and stored in memory.
Each light pulse launched into the fiber under test generates a set of sample data points representing the power of the reflected light at different locations along the length of the fiber. Since variable noise accompanies the generation of each set of sample points, one set of sample points will vary from another set of sample points. In order to obtain trace data points for a "smooth" OTDR trace, the sets of sample points are "ensemble" averaged with each other by averaging corresponding sample points of the different sets with each other. The greater the number of sets of sample points, the smoother the ultimate OTDR trace obtained by ensemble averaging. Often, to perform an OTDR test as described above, several hundred thousand sets of sample points must be generated and ensemble averaged with each other. To detect a fault in the network, the results of a "current" OTDR test are compared to a stored OTDR reference trace.
The foregoing OTDR testing technique suffers from several drawbacks. Since every trace data point is stored in the memory, the storage capacity of the memory must be relatively large to accommodate the amount of information stored therein. In addition, for each fiber, the time for taking the OTDR test may range anywhere from fifteen seconds to three or more minutes. This amount of time is required to obtain an OTDR trace which is accurate enough for comparison with an OTDR reference trace. If there are hundreds or thousands of fibers in the fiber optic network, the amount of time for testing the entire network is quite large. If a particular fiber contains a break or is degrading optical performance, detection of the problem may occur long after the occurrence of the problem.
Consequently, a need exists for a method and apparatus for scanning a fiber optic network which overcomes the aforementioned shortcomings associated with existing OTDR testing techniques.